Beam scanning galvanometers are used in many applications that require controlled high speed cyclical deflection of a beam of light across a defined field of observation to either produce or to read an image. Generally these galvanometers are designed to operate in either a resonant mode or a non-resonant mode.
Galvanometers that are designed to operate in a resonant mode are typically used in applications in which it is desired to use relatively low levels of power to drive the galvanometer. In these applications, an oscillating mirror of the galvanometer is supported with a flexure or spring. The stiffness of the spring and the mass and shape of the mirror produce a natural frequency for the galvanometer. When the mirror is oscillated at the natural frequency (i.e., resonant operation) very little power is needed to drive the mirror. When a galvanometer operates in a resonant mode, the mirror deflects symmetrically about a neutral position. In other words, the mirror deflects in a first direction to a first limit, returns to the neutral position and then deflects in a second direction to a second limit. Each of the limits are equidistant from the neutral position and the mirror consumes equal time in reaching either of the two limits.
In the resonant mode of operation, the angular displacement of the mirror follows a sinusiodal pattern as a function of time. A light beam that is reflected onto a flat surface by such a mirror follows a non-linear deflection pattern as a function of time. Thus, galvanometers that operate in a resonant mode are typically used only in applications which do not require linearity in the speed of a beam of light on a scanned surface. Additionally, these galvanometers are typically used when it is desired to perform a scanning operation during deflections of the mirror in either direction. This bidirectional scanning produces certain image distortions known as a "pincushion effect". Consequently, these resonant galvanometers are used only in applications which produce or read images on a low resolution basis, e.g., in the order of 200 dots per inch or less. In low resolution applications, the inherent non-linearity and distortions which arise from resonant scanning are acceptable.
Many systems that employ beam scanning galvanometers are not tolerant of the non-linearity of a resonant mode of operation of the galvanometer.
In some light scanning applications, such as laser printing, there is a need to transfer a relatively high concentration of energy with a scanning mirror. For example, in the thermal printer disclosed in previously cited U.S. Pat. application Ser. No. 457,593, a laser is employed to cause a transfer of dye from a donor dye film to a receiver. Presently, the power which a laser can introduce to this thermal printing system is limited to about 50 milliwatts. In order to produce a thermal printer which operates at the fastest possible rate within this power limitation of the laser, it is desirable to avoid scanning the laser beam across the dye donor film with a sinusoidal velocity. If a resonating beam scanning galvanometer (which produces a scanned beam with a sinusoidal velocity) were to be employed in this thermal printer, the speed of operation of the thermal printer would need to be reduced. This is because a beam traversing with a sinusoidal velocity has a peak velocity that is substantially higher than the average velocity. In a thermal printer, the scanning rate of the galvanometer (i.e., the number of scans per second) must be kept low enough so that the laser can produce the desired dye transfer at the maximum velocity of the scanned beam. In other words, if the 50 milliwatt laser will produce the desired dye transfer at a maximum beam velocity of 20 inches per second, then the rate of oscillation of the mirror must be kept low enough so that the maximum velocity of the beam is 20 inches per second. In a sinusoidal velocity profile, a maximum beam velocity of 20 inches per second corresponds to an oscillation rate that produces an average beam velocity of only about 12 inches per second. Thus a thermal printer using a sinusoidal beam velocity cannot achieve an optimum speed of operation for a particular power level of a laser used as part of the printer.
An optimization of speed of operation of the thermal printer for a particular power level of a laser is achieved when the beam is scanned across a workpiece at a constant velocity. In this mode of operation, the maximum velocity of the beam is equal to the average velocity of the beam. Thus the laser can be employed to deliver its maximum power during the entire scan cycle.
Galvanometers which are capable of scanning a beam of light across a flat surface at a constant velocity are known in the prior art. See, for example, U.S. Pat. No. 4,791,591 (K. Asanuma et. al.), issued Dec. 13, 1988. Galvanometers of this type (i.e., non-resonant) are operated in conjunction with a driving current control circuit. Typically these current control circuits function on a feedback principle.
The deflecting mirror of some galvanometers is equipped with an angular position sensing mechanism that is responsive to the actual deflection of the mirror. Driving current is continuously varied in response to the position sensing mechanism to assure that the mirror does indeed follow a desired deflection pattern as a function of time.
Typically these angular position sensing mechanisms consist of elements that produce variations in capacitance as a function of angular displacement. In order to operate effectively, two elements must move relative to each other in a concentric manner. One element moves with the mirror and the other element remains stationary. The elements must be very close to one another, in the order of 0.001 inches or less. In order to achieve such a closely spaced concentric motion between the sensing elements, the mirror must be rotated about a rigidly fixed axis. Consequently, the use of a capacitor-based angular position sensing mechanism in a galvanometer places a limitation on the type of mirror deflection which can be used in the galvanometer. In prior art non-resonant galvanometers, an axis of rotation of the mirror is supported in bearings (ball-bearings, needle bearings, sleeve bearings and etc.) in order to assure that the position sensors function properly.
Bearings that support a mirror in a galvanometer are subject to wear, particularly in the context of high speed operation that may require thousands of oscillations per minute. Bearings are also undesirable in a galvanometer because they permit a certain amount of deflection of the mirror in planes that are not parallel to the surface being scanned (i.e., cross-axis deflection). This cross-axis deflection can be controlled to a tolerable level with high quality bearings for applications that require moderately high resolution, i.e., 600 dots per inch or less. However, cross-axis deflection must be more carefully controlled in high resolution applications, i.e., 2000 dots per inch or higher.
Galvanometers which use bearing supported mirrors to produce high resolution images are relatively expensive. Such galvanometers typically cost between $500.00 and $1,000.00 per unit. Galvanometer costs this high are prohibitive when it desired to use galvanometers in thermal printers that are intended for desk-top applications. This is one reason that relatively low cost desk-top, high resolution thermal printers have heretofore not been produced with galvanometer-based printing systems.
An additional problem arises when galvanometers are used in a non-resonant mode in a unit such as a thermal printer. The galvanometers must be driven with forces substantially greater than those used to drive a galvanometer of an equivalent size in a resonant mode. These larger forces present two problems. First, the application of large forces requires the expenditure of large amounts of power, and second, the large driving forces produce equivalently large reaction forces that cause undesirable vibrations which interfere with the production of high resolution images. Each of these two conditions can be accommodated in a thermal printer, but typically at an undesirably high cost.
A thermal printer that provides a large amount of power to its galvanometer is typically too expensive for desk top applications. Thermal printers which consume large amounts of power are also less likely to be adaptable to applications where portability is a consideration.
Large reaction forces, that arise from a nonresonant mode of operation, can be prevented from producing undesirable vibrations by mounting the galvanometer on a rigid and massive support. However, introduction of massive support structures into a thermal printer precludes the use of the thermal printer in applications in which portability is an important consideration.
Heretofore, efforts were made to control the forces required to operate beam scanning galvanometers in a non-resonant mode. These efforts have focused on keeping the rotational axis of the mirror as close as possible to a reflecting surface of the mirror. When this distance is kept small, the moment of inertia of the oscillating mirror is kept correspondingly small and the forces needed to drive the mirror are thus kept relatively low. Prior art non-resonant galvanometers designs have heretofore employed bearings to support an axis of rotation of the mirror in order to maintain a low moment of inertia for the mirror.
It has been recognized that, from the point of view of reduced cross axis deflection, galvanometer mirrors supported on flexures or springs are superior to galvanometer mirrors supported on bearings. See for example, U.S. Pat. No. 4,732,440 (Gadhok), issued Mar. 22, 1988. However, the prior art does not teach how to apply the principles of a flexure-based mirror supporting system to a galvanometer that operates in a non-resonant mode.
An additional shortcoming of prior art nonresonant galvanometers is the technique used to achieve a scan-retrace mode of operation. When non-resonant galvanometers are used to perform printing operation, the scanning of a light beam is typically limited to one direction of travel of the mirror. In other words, a light beam is turned on while a mirror deflects in a first direction. While the mirror is being returned to a starting position, the light beam is turned off. This technique is known as a scan-flyback or scanretrace mode of operation. In order to avoid a waste of time during the flyback or retrace portion of a mirror deflection cycle, the mirror is typically returned to its starting position in a period of time which is much shorter than the scanning time.
Many prior art galvanometers are provided with springs that cause the mirror to rapidly return to its starting position after completion of a scan. These springs do not support the mirror. They are used only to drive the mirror to its starting position. Typically the mirror is driven to the return position at a very high speed. A mirror driven in such a high speed manner returns to its starting position with a shock that produces vibrations in the mirror and the spring. When a scan cycle begins immediately after the mirror reaches its return position, these vibrations adversely effect the stability of the beam scanning. In moderate resolution printing these vibrational anomalies are sufficiently small as to have only second order effects on the quality of an image. However, in printing operations requiring resolutions of 2000 dots per inch and higher, these vibrations are intolerable.
It is desirable therefore to provide a beam scanning galvanometer that is usable in high resolution printing operations which is free from undesirable bearing support systems and anomalous vibrations and which operates with relatively low peak power requirements.